Universal Mobile Telecommunications System (UMTS) is one of the third-generation (3G) mobile telecommunications technologies. Currently, the most common form of UMTS uses Wideband Code Division Multiple Access (W-CDMA) as the radio access technology. UMTS is standardized by the Third Generation Partnership Project (3GPP). UMTS, using W-CDMA, supports up to 21 Mbit/s data transfer rates with HSDPA (High Speed Data Packet Access).
In 3GPP, work is ongoing on specifications of the UMTS Terrestrial Radio Access Network (W-CDMA) evolution (E-UTRA) as part of the Long Term Evolution (W-CDMA) effort. In forthcoming evolutions of cellular system standards like LTE the maximum data rate will increase. Higher data rates typically require larger system bandwidths. However, since radio spectrum is a limited resource and since many operators and systems need to share the same radio resource, there is very complicated to find a large amount of contiguous free spectrum, e.g. 100 MHz.
One method to overcome this problem is to aggregate non-contiguous spectrum and thereby from a baseband point of view a large system bandwidth is created. The benefit with such a solution is that then it will be possible to generate sufficiently large bandwidth for supporting data rates up to (and above) 1 Gb/s.
Furthermore, such a scenario also makes it possible to adapt the spectrum parts to the current situation and geographical position making such a solution very flexible. A straightforward evolution of current cellular systems, like LTE, to support non-contiguous spectrum is to introduce multi-carrier. That means, for each spectrum, a “chunk” represents a “legacy” system, i.e. a single carrier system, and the future multi-carrier mobile terminals will be capable to receive multiple number of legacy carriers of different bandwidths transmitted at different carrier frequencies.
A discontinuous reception (DRX) mechanism is a mechanism allowing the UE to stop monitoring layer 1/layer 2 (L1/L2) control signaling channels, which allows the UE to e.g. turn off some or all of its radio circuitry, to decrease the power consumption. The DRX is applicable when a UE has an established RRC connection (i.e. when UE is in RRC13 CONNECTED state).
DRX in LTE specifies two pre-defined cycles, a long DRX cycle (longDRX-Cycle) and a short DRX cycle (shortDRX-Cycle). When DRX is configured, the network always configures the UE with the long cycle and may optionally configure the UE with the short cycle, in which case the short cycle always is a fraction of the length of long cycle.
At the beginning of the DRX cycle, the UE shall monitor the Packet Data Control Channel (PDCCH) over a certain amount of Transmission Time Intervals (TTIs); this is also referred to as the DRX on-duration period which is controlled by the DRX OnDurationTimer. The beginning of the cycle is determined by the system frame number (SFN), specified as an integer offset of the DRX start offset.
FIG. 1 shows an example of a DRX cycle pattern, i.e. the periodic repetition of the DRX on-duration period followed by a possible period of inactivity.
The transition from the short cycle to the long cycle occurs after a period of consecutive TTIs for which the UE has not been scheduled using the PDCCH by using the drxShortCycleTimer. At most one DRX cycle is active at any given time.
The PDCCH carries downlink scheduling assignments as well as uplink scheduling grants. When the UE successfully decodes PDCCH, it starts (or restarts) the DRX Inactivity Timer (drx-InactivityTimer) and monitors PDCCH until the timer expires. It also starts a HARQ RTI Timer for the relevant HARQ process, to handle possible retransmission; when the HARQ RTT timer expires and the data was not successfully decoded, the UE starts the DRX Retransmission Timer (drx-RetransmissionTimer) when it monitors the PDCCH. Whether the UE is awake (i.e. monitors the PDCCH) or asleep (i.e. not monitors the PDC) after the DRX on-duration period thus depends on the scheduling activity for the UE, i.e., it depends on the reception and successful decoding of the PDCCH control signaling during the period when the UE is already monitoring PDCCH i.e. the DRX Active Time.
The DRX Active Time includes the time while the DRX On Duration Timer (OnDurationTimer) or the DRX Inactivity Timer (drx-InactivityTimer) or a DRX Retransmission Timer (drx-RetransmissionTimer) is running. Note that the Active Time also includes subframes during contention resolution for random access, subframes while a scheduling request is pending and subframes during which an uplink grant for a pending HARQ retransmission can occur and other prescriptions as described in TS 36.321 subclause 5.7.